In its natural state, “raw” or “sour” natural gas contains acid gases such as carbon dioxide (CO2) and hydrogen sulphide (H2S). The process to produce pipeline quality natural gas requires the removal of these naturally occurring gases, typically through a liquid absorption process. Conventional acid gas absorbing liquids commonly used in the industry include amine-based solutions, or liquid amine.
The chemical reactions between the amine solution and the acid gas are reversible, allowing for thermal regeneration of the “rich” amine solution to remove the CO2 and H2S. The regenerated “lean” amine solution is then reused for another acid gas absorption cycle. The thermal regeneration of the “rich” amine solution is the single most energy intensive step during the acid gas removal process.
Gas processing plants do not currently have a means of continuously measuring the acid gas concentrations in the loaded (rich) and the regenerated (lean) absorbing liquid. The acid gas (CO2 and H2S) loading of the absorbing liquid during the process is conventionally determined manually by a plant operator based on lab titration. In most cases, an excess amount of energy is used to regenerate the absorbing liquid in order to meet pipeline gas specifications. As a result, the absorbing liquid circulation rate and thermal regeneration temperatures are operated with a wide margin and are not optimized. The ability to measure the absorbing liquid acid concentration continuously, especially for the regenerated lean liquid, would be useful to the natural gas processing industry.
The inventors have identified that Raman spectroscopy can be used to measure the acid gas loading of the liquid absorption process. Raman spectroscopy is a spectroscopic method to study the chemical components in gas, liquid or solid state phases through the vibration or rotation of a molecule. Raman spectroscopy is commonly used to characterize chemical components by providing a fingerprint by which the molecule can be identified. Typically, a sample is illuminated with a light source in which the light is collected with a lens and sent through a monochromator. Wavelengths close to the laser line (due to elastic Rayleigh scattering) are filtered out and those in a certain spectral window away from the laser line are dispersed onto a detector. Spontaneous Raman scattering is typically very weak and, as a result, the main historical difficulty of employing Raman spectroscopy has been separating the weak inelastic ally scattered light from the intense Rayleigh scattered laser light. A Raman spectrometer consists of three main parts: a light source, a spectrograph and a detector.
The inventors are not aware of any previous application of Raman spectroscopy to the measurement of acid gases in a basic solution, such as an amine solution used in a gas processing plant.